The division operator in Python 2 on two integer arguments, which your lengths are, will do integer division. In integer division the fractional part is discarded. In Python 3 this was changed to do floating point division.

There are two solutions:

The first is to cast the arguments to a float:

AT_content = float(a_count + t_count)/(lenght_DNA)

the second is to change the division behaviour to the Python 3 behaviour at the top of your script: